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Structural changes of (Ag2O)x(B2O3)1-x glasses, over the range between X = 0.0 and X = 0.14, have been monitored by Raman spectroscopy. Low temperature heat capacity measurements have been also performed between 0.4 K and 25 K to investigate along with low frequency Raman scattering the behavior of the low-energy vibrational dynamics. A progressively decreasing intensity of the main band at 808 cm−1 and the appearance of a band at about 775 cm−1 with increasing Ag2O content have been observed and analyzed in detail. The strongly polarized band at 808 cm−1 is ascribed to the breathing vibration of boroxol rings and the decrease of its intensity indicates the reduction of the number of these units in the network. The band at 775 cm−1 is assigned to vibrations of pentaborate units, formed from two boroxol rings linked by a tetrahedral BO4 group. The rate of formation of BO4 groups appears to increase as X/(1-X). Raman investigation has been extended to very low frequencies, where a broad intense band, the Boson peak (BP), dominates the spectrum of all the samples below 100 cm−1. Addition of Ag2O shifts the BP position from 26 cm−1 up to 33 cm−1 and gives rise to a decrease of its intensity up to X = 0.09 and a sudden increase for X = 0.14, which has been explained as due to rattling motion of silver ions whose contribution is added to that of localized vibrations. The formation of tetrahedral BO4 groups stiffens the network and leads to a decrease of the excess heat capacity over the Debye prediction below 20 K, which is not taken into account by the simple hardening of the elastic continuum but seems to follow the reduction of boroxol rings. This result contrasts the observations on borate glasses with high Ag content and emphasizes the crucial role of the network continuity on the vibrational dynamics of a glass.

The specific role of acetonitrile and methoxypropionitrile, as accelerators of the relaxation dynamics of polyvinyl butyral (PVB), was investigated in polymer/additive mixtures with a saturation liquid content. The aim was to improve the ionic mobility of PVB-based solid electrolytes to be used in solid dye-sensitized solar cells. Mechanical and dielectric relaxation measurements between 120 K and 380 K revealed that the a-relaxation observed above 330 K in dry-PVB is shifted quite below room temperature in PVB/additives. Both the additives cause a growing intermolecular cooperativity, the sub-glass b-relaxation exhibiting a strength enhanced by a factor 3 and a frequency factor which increases from 1015 s21 to 1021 s21. This discloses an activation entropy as high as 165.7 J/K mol in comparison to 40.8 J/K mol in dry-PVB. It is suggested the existence of cooperative transitions, mainly driven by bridges formed through additive molecules, which influence both short- and long-scale segmental motions and also favor the ion dynamics in PVB/additive/electrolyte systems. The room temperature ionic conductivity rrt exhibits large changes from 6.4*10214 S/m in dry PVB, through 1.5*1028 S/m in PVB/LiI, to 2.45*1025 S/m in PVB/MPN/LiI.bject]

This study reports on characterization and photoactivity of nanostructured TiO2 samples, which have been permanently densified under high pressures, up to 2.1 GPa. Commercial Mirkat 211 anatase has been used as a benchmark sample, in order to investigate the effect of unidirectional high pressure on structural, optical and photocatalytic properties of TiO2. Vibrational Raman spectroscopy shows that the treatment does not cause transitions among the different crystalline phases of titanium dioxide. UV-vis diffuse reflectance spectra reveal that increasing pressure gives rise to a shift of the absorption onset towards higher wavelength enhancing the photoactivity under visible radiation. Samples are also photo-electrochemically characterized and tested in the gas phase with partial oxidation of ethanol to acetaldehyde under visible irradiation. Compaction up to 0.8 GPa depresses both the alcohol conversion and the aldehyde yield, while samples treated under higher pressures show enhanced characteristics of conversion compared to the pristine material. Moreover, promising results in the reduction of CO2 are also obtained under UV-visible radiation.

Measurements of differential scanning calorimetry, dynamic mechanical analysis, and dilatometry have been performed in heterocyclic polymer networks (HPNs), whose effective network density has been gradually varied keeping the overall chemical structure essentially unchanged. Evidence of a growing intermolecular cooperativity for a local relaxation motion is offered by the investigation of the subglass mechanical ?-relaxation, whose frequency factor and apparent activation energy strongly increase with increasing cross-linking density from about 1015 s-1 and 44.2 kJ/mol to about 1019 s-1 and 69.8 kJ/mol. The analysis of the characteristics of the mechanical ?-relaxation suggests the existence of cooperative transitions of consecutive relaxing units, mainly driven by the crank-shaft motion of the network junctions between the isocyanurate heterocycles building up the structure. Comparison with previous results concerning the dielectric ?-relaxation evidence a less cooperative character and a different microscopic origin for the conformational transitions driving the dielectric process. Moreover, a decrease in the thermodynamic fragility with increasing network density has been revealed, which contrasts with the increasing dynamic fragility, as measured by dielectric and mechanical probes.

The infrared absorption spectra of thermoplastic interpenetrating polymer networks based on semi-crystalline polyurethane (CPU) and a styrene/acrylic acid block-copolymer (S-b-AA(K+), salt form) have been measured over the spectral range between 4000 and 600 cm-1 in an attempt to study their structure. The analysis of the spectra of the individual polymers and CPU/S-b-AA(K+) blends has shown the mutual influence of components on the formation of a network of intra- and intermolecular physical bonds between functional groups of the components. In close agreement with these observations, the composition behaviors of experimental results of density, tensile strength, differential scanning calorimetry and dynamic mechanical spectroscopy reveal deviations from a simple additive law which have been associated to a weak affinity of the individual components. It has been suggested that the existence of specific weak interactions (H-bonds) between the functional groups of the two components ensure the formation of a mixed microphase.

Inelastic light-scattering spectra of normal and permanently densified ${\mathrm{B}}_{2}$O${}_{3}$ glasses were investigated over the frequency range between 6 and 1600 cm${}^{\ensuremath{-}1}$. Densification from 1826 to 2320 kg$/$m${}^{3}$ was obtained by loading ${\mathrm{B}}_{2}$O${}_{3}$ glasses in a multianvil apparatus for synthesis at 2 and 4 GPa; they were fused at temperatures between 1400 and 1500 K and then were quenched at those pressures. It is shown that increasing densification gives rise to (i) a growing decrease of the intensity of the strong band at 808 cm${}^{\ensuremath{-}1}$, ascribed to the breathing vibration of boroxol rings, implying a reduction in the number of rings in the network and (ii) the appearance of a band at 775 cm${}^{\ensuremath{-}1}$ in the glass compacted at 4 GPa, which is assigned to the vibrations of structural units containing tetrahedral BO${}_{4}$ groups. The low-frequency Raman scattering includes the boson peak (BP), which dominates the spectra between 10 and 100 cm${}^{\ensuremath{-}1}$. Densification significantly decreases the intensity of the BP and shifts its position from about 26 cm${}^{\ensuremath{-}1}$, through 39 cm${}^{\ensuremath{-}1}$ (2-GPa glass), up to 68 cm${}^{\ensuremath{-}1}$ (4-GPa glass). These increases are stronger than those expected from the substantial hardening of the elastic continuum: The elastic moduli increase up to about a factor of 5 compared to those of normal glass. These observations imply that densification drives the system toward a structure having a more efficient packing of molecular units, causing substantial variations of the short- and medium-range orders with the formation of boron atoms in the four-fold-coordinated state at a quenching pressure of 4 GPa.

Specific heat was measured over the temperature range 0.4–25 K in GeO2 glasses, compacted at pressures of 2 and 6 GPa to achieve more than 20% densification. Above 1 K, specific heat C was reduced by up to a factor of 2.5 with increasing density; the variation being stronger than that observed below 1 K. The broad peak observed above 1 K in a C/T 3 vs. T plot shifted to higher temperatures with increasing density, exhibiting changes which cannot be explained by the modifications of the elastic continuum. In addition, the peak showed no changes in shape, clearly indicating that the overall distribution of the underlying vibrational modes was independent of density. The temperature dependence of C over the explored range was in qualitative agreement with the predictions of the Soft Potential Model.

In this study, we present the dependence of the relaxation processes from the size cation and thermal history in complexes of polyethylene oxide (PEO) and alkali metal salts (sodium and potassium thiocyanates). The internal friction behavior as a function of temperature and frequency reveals the presence of local (γ) and cooperative (a a ) molecular relaxations. The region of the γ-relaxation, which shows a single mechanical loss peak in pure PEO and in PEO-NaSCN, is strongly modified by the addition of potassium salt to PEO and two close peaks are present. Samples, prepared by a slow evaporation of the solvent, have a semi-crystalline structure. A “quenching” procedure imposed on them gives rise to the same structure, but enhances the amorphous phase and shifts the glass transition to lower temperatures, probably as an effect of decreasing constraints of the crystalline phases on the amorphous regions. Finally, different thermal histoires also produce changes on the low temperature γ-relaxation, which are discussed accordingly with the revealed phase behavior.

International audience; Comparative measurements of Brillouin light scattering and ultrasounds in (K2O)0.04(B2O3)0.96 and (Ag2O)0.14(B2O3)0.86 borate glasses as a function of temperature between 1.5 and 300 K reveal that distinct mechanisms regulate the temperature behaviours of the acoustic attenuation. In the MHz range the attenuation and the sound velocity are mainly governed by (i) quantum-mechanical tunnelling below 20 K, (ii) thermally activated relaxations between 20 and 200 K and (iii) vibrational anharmonicity at even higher temperatures. In the GHz range and in the temperature interval between 77 and 300 K, additional contributions besides the relaxation process must be taken into consideration to account for the hypersonic attenuation.

The fragility and the anharmonicity of (Ag2O)x(B2O3)1−x borate glasses have been quantified by measuring the change in the specific heat capacity at the glass transition temperature Tg and the room-temperature thermodynamic Gruneisen parameter. Increasing the silver oxide content above X = 0.10 leads to an increase of both the parameters, showing that a growing fragility of a glass-forming liquid is predictive of an increasing overall anharmonicity of its glassy state. The attenuation and velocity of ultrasonic waves of frequencies in the range of 10–70 MHz have also been measured in silver borate glasses as a function of temperature between 1.5 and 300 K. The experimental data reveal anelastic behaviours which are governed by (i) quantum-mechanical tunnelling below 20 K, (ii) thermally activated relaxations between 20 and 200 K and (iii) vibrational anharmonicity at even higher temperatures. Evaluation of tunnelling (C) and relaxation (C*) strengths shows that C is independent of the structural changes affecting the borate network with increasing metal oxide content and is at least one order of magnitude smaller than C*. The latter observation implies that only a small fraction of the locally mobile defects are subjected to tunnelling motions.

The relaxation losses and the corresponding velocity variations, observed at ultrasonic frequencies in (M2O)0.14(B2O3)0.86 alkali borate glasses (M = Li, K, Cs) between 1.5 and 300 K, have been modelled by an asymmetric double-well potential model having a distribution of both the barrier potential and the asymmetry. It is shown that the relaxation strength C* and the spectral density of asymmetries f0 decreases markedly with decreasing cation size. Below 10 K the sound attenuation is regulated by the phonon-assisted relaxation of tunnelling systems and exhibits a tunnelling strength C, ranging between 10−4 and 10−3. At variance with the behaviour observed for C*, C slightly increases with decreasing cation size and is more than one order of magnitude smaller than C*. It is concluded that, differently from classical relaxing states, tunnelling systems are independent of bond strengths and of structural changes characterizing a glassy network, confirming their inherent universality. Above about 120 K the ultrasonic velocity is mainly regulated by vibrational anharmonicity and shows a nearly linear decrease as the temperature is increased, the slope scaling with the cation size. Taken together, the observations point to the existence of a distinct correlation between anharmonicity and local mobility in the glassy network.

The attenuation and velocity of ultrasonic waves of frequencies in the range of $10\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}70\phantom{\rule{0.3em}{0ex}}\mathrm{MHz}$ have been measured in ${({\mathrm{Li}}_{2}\mathrm{O})}_{x}{({\mathrm{B}}_{2}{\mathrm{O}}_{3})}_{1\ensuremath{-}x}$ borate glasses as a function of temperature between 1.5 and $300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Two distinct features characterize the attenuation behavior: (i) a plateau at temperatures below $10\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, (ii) a broad high-temperature peak. The former feature is interpreted in terms of the phonon-assisted relaxation of two level systems and the latter by assuming the existence of a distribution of thermally activated relaxing centers. The spectral density of two-level systems results to be independent on the alkali oxide content, while the density of relaxing particles decreases with increasing lithium ion concentration supporting their association to the triangular $\mathrm{B}{\stackrel{\ensuremath{\llap{\not\;}}}{\mathrm{O}}}_{3}$ units building up the borate skeleton ($\stackrel{\ensuremath{\llap{\not\;}}}{\mathrm{O}}=\text{oxygen}$ atom bridging between two network-forming ions, i.e., boron ions). The comparison between the number densities of two-level systems and of relaxing particles indicates that only a small fraction of the locally mobile defects are subjected to tunneling motions. At temperatures below $100\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ the sound velocity is mainly governed by the relaxation contribution, while above $100\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ it is regulated by the vibrational anharmonicity and shows a nearly linear temperature dependence, whose slope strongly depends on the concentration of network modifier ions (${\mathrm{Li}}^{+}$ ions). The determination of the average thermal Gr\"uneisen parameters permits us also to disclose the existence of a distinct correlation between anharmonicity and fragility of lithium borate glasses: a growing fragility is predictive of an increasing anharmonicity.

The attenuation and velocity of ultrasonic waves of frequencies in the range from 10 MHz to 50 MHz have been measured in (M 2 O) 0.14 (B 2 O 3 ) 0.86 borate glasses (M = Li, Cs or Ag) as a function of temperature between 15 K and 300 K. Broad relaxation peaks characterize the acoustic attenuation at temperatures below 100 K and are paralleled by corresponding dispersive behaviors of the sound velocity. The velocity of sound waves decreases with increasing temperature in all the glasses, the decrease as the temperature is increased being larger in glasses containing cesium than in those with lithium or silver. Above 100 K the ultrasonic velocity is regulated by the vibrational anharmonicity and shows a nearly linear temperature dependence, whose slope scales with the size of the network modifier ion. The results suggest that the local motion of structural defects and the anharmonicity in borate glasses are strongly affected by the polarizing power of network modifier ions.